The present invention relates to a fluid flow control device and more particularly, but not exclusively, to such a device that is used to control the flow of pneumatic or hydraulic fluid to an actuator of the kind used in a valve positioner.
In many applications it is desirable to automate the actuation of a pipeline valve via a remote control system. This is particularly necessary in harsh environments such as, for example, a petrochemical pipeline located on land or off-shore. The operation of, for example, a ball valve in such a pipeline is often effected by a valve positioner that has a fluid (e.g. pneumatic) actuator for operating the valve. It is often necessary to employ a volume booster to ensure that there is a sufficient sustained volume of fluid (e.g. air) available to the actuator to ensure a rapid response time. The actuator may, for example, take the form of a pneumatically-operated piston and cylinder assembly. The volume booster operates to ensure that the pressure of the supply fluid to the actuator is regulated and the volumetric flow is sustained to achieve the desired actuator stroke speed. Separate flow regulators are often connected to the booster and this serves to increase the complexity of the system in terms of installation, servicing, maintenance and operation.
Volume boosters are generally controlled by a pneumatic pilot signal received from a control part of the valve positioner. The signal is generated in response to a command signal directing the positioner to move the valve to a desired position. The command signal may be an open-loop signal or a closed-loop feedback electrical control signal that takes into account the position of the actuator. In an alternative arrangement the booster may be controlled directly by an electrical signal that operates a solenoid valve in the booster.
A conventional form of volume booster comprises a housing having an operating air inlet and outlet, both of which are in communication with the flow of operating air to the actuator, and a pilot signal inlet connected to the output of the positioner. The communication between the operating air inlet and outlet is selectively interrupted by a supply valve whose position is controlled by diaphragm assembly on which the pilot signal acts. The supply valve is connected to one end of a reciprocal valve stem the other end of which serves to open or close an exhaust valve in an exhaust passage defined in the diaphragm assembly. The pilot signal acts on one side of the diaphragm assembly whereas the outlet air pressure acts on the other side of the diaphragm assembly by virtue of a bleed passage in the housing from the outlet. In the event that force of the pilot signal pressure applied to the first side of the diaphragm assembly exceeds that applied on the other side by the outlet pressure, the force differential serves to move the diaphragm assembly and valve stem to a first position in which the supply valve is open and the exhaust valve remains closed. Operating air can then flow from inlet to outlet so as to drive the actuator and position the valve. When the outlet pressure increases or the pilot signal pressure decreases to the extent that the forces on the diaphragm assembly cause it to move in the opposite direction, the diaphragm assembly moves to a second position in which the supply valve is closed and the diaphragm assembly lifts off the exhaust valve so that excess pressure can vent between the diaphragm assembly and the exhaust valve to the exhaust passage in the diaphragm assembly. The exhaust valve may be defined by a simple poppet valve on the end of the valve stem that seals against a seat defined at a bore in the diaphragm assembly. The location of the exhaust valve and passage means they tend to be relatively small and thus serve to restrict flow. The flow rate is significantly lower than that of the main flow leading to a slow reaction time. This is particularly undesirable in the event of an emergency where it is necessary to vent large volumes of air.
One solution to the problem of restricted exhaust flow is to provide a separate exhaust flow having a capacity equivalent to the main operating air flow. This involves additional components, space and expense. These may be provided in the same housing as the booster or as a separate component.
In one example of a separate exhaust capacity, an external conduit disposed outside the main body of the booster housing interconnects the outlet and the exhaust passage which are provided on opposite sides of the diaphragm assembly. An example of this is pneumatic volume booster Model 200XLR available from Fairchild Industrial Products Company of Winston-Salem, NC, USA. Without the restriction imposed by the space within the body of the booster, the external conduit can have a relatively large size so as to permit the exhaust flow to be as large as the main flow. This solution is relatively large and cumbersome and can therefore be disadvantageous in applications where there are space constraints.
It is an object of the present invention to obviate or mitigate the above, and other, disadvantages. It is also an object of the present invention to provide for an improved, or alternative, fluid flow control device.